Method of growing semiconductor rods from a pedestal

ABSTRACT

A METHOD FOR GROWING SEMICONDUCTOR RODS FROM A SELFSUPPORTING BLOCK OF MATERIAL WHEREIN THE FORMATION OF A RIM AROUND THE PERIMETER OF THE BLOCK OF MATERIAL IS PREVENTED BY BOMBARDING THE UPPER SURFACE OF THE BLOCK OF MATERIAL WITH AN ANNULUS OF ELECTRONS WHOSE CENTER IS OFFSET FROM THE CENTER OF THE UPPER SURFACE OF THE MATERIAL, THEREBY CAUSING EVERY PORTION OF THE UPPER SURFACE TO PERIODICALLY BE HEATED BY SAID BEAM OF ELECTRONS. THE ROD IS DRAWN FROM A POOL OF MELTED MATERIAL ON THE UPPER SURFACE OF THE BLOCK OF MATERIAL AND APPROXIMATELY AT THE CENTER OF THE AMMULUS OF ELECTRONS.

Dec. 14, 1971 -r. F. clszEK' METHOD 0]: GROWING SEMICONDUCTOR RODS FROM A PEDESTAL Filed April 5, 1969 *7!- (PRIOR ART INVENTOR THEODORE F. CISZEK ATTORNEY United States Patent US. CI. 23-30] SP Claims ABSTRACT OF THE DISCLOSURE A method for growing semiconductor rods from a selfsupporting block of material wherein the formation of a rim around the perimeter of the block of material is prevented by bombarding the upper surface of the block of material with an annulus of electrons whose center is offset from the center of the upper surface of the material, thereby causing every portion of the upper surface to periodically be heated by said beam of electrons. The rod is drawn from a pool of melted material on the upper surface of the block of material and approximately at the center of the annulus of electrons.

BACKGROUND OF THE INVENTION The present invention relates to rod or crystal growth and more particularly to a method of preventing the formation of a rim around the perimeter of a block of material from which a rod is being drawn.

Monocrystalline or single crystal rods of semiconductor materials are of great value to the electronics industry. As the use of these monocrystalline rods has expanded, less expensive methods of producing them and methods of reducing the impurities in them have been sought.

In the past, monocrystalline rods have been grown by melting a block or charge of the desired material in a crucible, dipping a suitable seed crystal into the molten material and drawing that seed crystal out from the molten material at a rate sufficient to draw or grow a crystal. This method is widely known as the Czochralski method for growing crystals, and numerous variations and embodiments of this method have been patented or published in scientific journals.

However, the use of a crucible to contain the molten material has a disadvantage in that impurities, such as oxygen, can diffuse into the molten material from the heated crucible wall. Additionally, for those materials that melt at high temperatures, such as silicon and germanium, difficulties arise due to the fact that the crucible walls become plastically deformable at those high temperatures.

Accordingly, various methods have been attempted or developed for growing crystals from a self-supporting block of material to avoid the contamination caused by growth from a crucible. More particularly, US. Pat. 2,858,199, issued Oct. 28, 1958, to C. C. Larson, discloses the method of growing crystals from a pedestal charge of some suitable material which charge serves as both the source of material to be melted and a crucible for the melted material.

Briefly stated, Larsons method is to bombard the central portion of the upper surface of a block of suitable material with an electron beam until a pool of melted material forms in the center of the upper surface and is contained within the perimeter of the block by the unmelted portion of that upper surface. A suitable seed crystal is then immersed in the pool of melted material and pulled from the pool at a rate which allows solidification of the melted material uplifted from the pool on the seed crystal.

After the seed crystal has been drawing molten material ice from the center of the pedestal charge for some time, a substantial portion of the charge has been removed, and a hole or void is formed in the center portion of the charge. From another point of view, the pedestal charge can be said to have grown a rim. It has been found that the growth of such a rim on the pedestal charge causes several problems in the growth of crystals by the above-cited method of Larson.

The unmelted rim portion of the pedestal charge cannot efficiently be remolded into a new charge for subsequent use. Thus, this expensive raw material is wasted. Further, the diameter and the length of the crystal that can be drawn from such a pedestal is necessarily limited by the width of the unmelted rim portion of the charge that is required to confine the melted portion of the charge without melting away itself. Additionally, it will be realized that for a given pedestal charge of any size, as the total amount of that charge melted and added to the pool of melted material is increased, the size of the crystal that can be drawn from that pool is also increased. Eliminating the growth of an unmelted rim on the charge increases the amount of material added to the pool.

Thus, to grow the largest crystal possible from a given pedestal charge and to make the most efficient use of the raw materials, the crystal pulling apparatus, the power required to raise the charge to its melting point and the time required for starting and terminating each crystal growth, no rim growth on the pedestal charge can be allowed.

Still another problem relates to the fact that the thermal field present in the pedestal charge and the pool of melted material directly affects the growth of a crystal and must be carefully controlled. It has been found that the growth of a rim on the pedestal charge causes the thermal field to vary significantly during crystal growth. This variance has been found very difficult to control and frequently has caused the crystal growth to be aborted.

Finally, in order to follow and control the variables involved in the growth of a crystal, it is necessary to be able to optically monitor the crystal-pool interface at nearly all times. This monitoring is necessary for either manual or automated crystal growth. However, when a rim is allowed to grow on the pedestal charge, visibility of the crystal-pool interface is first impaired and then eliminated as that interface gradually recedes into the pedestal charge.

SUMMARY OF THE INVENTION Another object is to provide a method of growingsemiconductor rods from a pedestal charge which method provides for optical monitoring of the rod-pool interface at all times and provides a nearly constant thermal field in the pedestal charge and pool of melted material.

In accordance with these and other objects, there is provided by the present invention a method for the growth of either monocrystalline or polycrystalline semi-conductor rods from a pedestal charge and the simultaneous prevention of the formation of a rim of unmelted material around the perimeter of that pedestal charge. The pedestal charge is first caused to continuously rotate about its vertical axis. Then, an annular beam of electrons is directed onto the upper surface of the pedstal charge so that the center of the annulus defined by the intersection of the annular beam and the upper surface of the pedestal charge is at an offset or different location than the center of the upper surface of the pedestal charge. Further, a portion of the annulus extends to and is substantially in juxtaposition with a portion of the perimeter of the upper surface and another portion of the annulus also extends beyond or is diametrically on the opposite side of the center of the upper surface from that side on which the center of the annulus lies.

Thus, every portion of the entire upper surface of the pedestal charge is periodically exposed to the heating effect of the annular beam of electrons and caused to become a part of the pool of melted material thereon. A seed crystal is then immersed into and drawn from the pool in the vicinity of the coolest portion of the pool, which generally is in the vicinity of the center of the annulus of electrons. This procedure is then continued until the pedestal charge is substantially consumed and results in the growth of a semiconductor rod.

BRIEF DESCRIPTION OF THE DRAWING Other objects and advantages will become obvious to those skilled in the art by reading the following detailed description in connection with the accompanying drawings wherein:

FIG. 1 is a vertical elevation partly in section of an embodiment of this invention;

FIG. 2 is a perspective view of a cylindrical block of material from which a rod is being drawn according to the method of the present invention; and

FIG. 3 is a vertical sectional elevation of a prior art pedestal charge showing the formation of a rim thereon.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings wherein like reference numerals designate like or corresponding parts throughout the figures thereof, there is shown in FIG. 1 a semiconductor rod growing furnace comprising an evacuated container which is hermetically sealed to a base 13. The container 10 can be made of quartz or stainless steel and can have a connecting tube 11 for evacuation of the container and a glass window 12 through which the method of the present invention can be observed when carried out inside the container 10.

Mounted within the container 10 on a pedestal 15 which is preferably maintained at ground level electrical potential and is secured to the end of a reciprocably and rotatably mounted shaft 16, as indicated by the pertinent arrows, is a self-supporting block of material 17, or pedestal charge, from which a semiconductor rod is to be grown. In particular, the self-supporting block of material 17 can be a block of silicon, germanium or aluminum oxide.

The upper surface of the block of material 17 underlies and supports a pool of melted material 19 which was formed by bombarding the upper surface of the block of material 17 with an annular beam of electrons 21 emitted from an electron beam gun 23. The beam of electrons 21 is properly focused on the upper surface of the block of material 17 by an electromagnetic or magnetic focusing coil. Such coils are well known for that purpose. The gun 23 and coil 27 are mounted within the container 10 by conventional mounting means. Obviously, more than one such focusing coil can be used if desired or advantageous.

Although FIG. 1 does show an annular electron beam gun emitting an annular beam which is focused by an annular focusing coil, it is to be understood that the method of the present invention can be practiced using several individual electron guns and magnetic or electromagnetic focusing coils properly spaced to form an annular beam. Likewise, a suitable annular electron beam can be obtained by sweeping the beam from one electron gun across the upper surface of the block of material 17 in an annular pattern. For the purposes of this application, the term annular beam is intended to include both a beam which is continuously annular and a beam which is substantially continuously annular.

By modifying the supporting structure to provide a hollow center therein, a specific electron gun suitable for performing the method of the present invention is the self-accelerated electron beam gun described by H. R. Smith, Jr. in the book, Introduction to Electron Beam Technology, Robert Bakish ed., John Wiley and Sons, Inc., New York, N.Y., chapter 7, page 176, fig. 7.5, 1962.

Returning to FIG. I, inserted through the center of the gun 23 and coil 27 is an upper holder 33 which is secured to the end of a reciprocably and rotatably mounted shaft 35, as indicated by the pertinent arrows. This shaft 35 serves as the means for drawing a seed crystal and the growing rod 31 from the pool of melted material 19.

The shafts -16 and 35, the holder 33 and the pedestal 15 can be made of stainless steel. Further, the shafts 16 and 35 are held in an air-tight or vacuum proof relationship to the container 10 by gaskets, fittings, O-rings, or the like, 41 and 43 respectively. Suitable means for rotating and/ or reciprocating the shafts 16 and 35 and therefore the pedestal 15 and upper holder 33, are well known to those in the crystal growing art and are described in great detail in several prior art patents.

It is well known in the art of crystal growing that crystal contamination can be greatly reduced by growing the crystal in a vacuum. Further, the operation of an electron beam requires a vacuum of about 10* torr within the container 10. Thus, the pedestal charge or block of material 17 must be a material which is at least slightly electrically conductive, and it must not be a material which, when in a molten state under a pressure of at least 10- torr, will vaporize to the extent that the pressure in the container 10 will be raised above 1O torr by that vaporization. Silicon, germanium and aluminum oxide can meet that requirement and, therefore, are suitable materials for crystal growth by the method of the present invention.

When a monocrystalline or polycrystalline semiconductor rod is to be grown, the block of material 17 is placed in the container 10 and adjusted to a desired level by raising or lowering the pedestal 15. The container 10 is then evacuated by means of a suitable vacuum pump through connecting tube 11. The block of material 17 is then rotated continuously in either a clockwise or counterclockwise direction by the similar rotation of the pedestal 15.

The block of material 17 can be preheated by radiant heating, for example. But preferably, the electron beam gun 23, or another electron gun mounted within the container 10, can be used to heat the upper surface of the block 17 to its melting point.

Once the upper surface of the block 17 begins to melt, the electrons emitted from the gun 23 are directed onto the upper surface of the block 17 as an annular beam. As the block of material 17 continuously rotates about a vertical axis which intersects its upper surface at C, every portion of its upper surface is subjected to the heating effect of the annular electron beam 21 and melts to form the pool of melted material 19, as illustrated in FIG. 2. The melted material forms an eccentric pool 19 inside of the perimeter of the upper surface of the block 17 and covers nearly all of that upper surface.

As described hereinabove, previously known methods of growing a crystal from a pedestal require the growth of a rim 51 around the perimeter of the pedestal charge 53 as illustrated in FIG. 3 to contain the pool of melted material. To completely obviate the problems already listed herein that are caused by the growth of the rim 51, not to mention those problems which occur when that rim caves in, the annular beam of electrons 21 is so directed onto the upper surface of the block of material 17 that the center B of the annulus 61 defined by the intersection of the annular beam of electrons 2-1 and the upper surface of the block of material 17 is offset from the center C of that upper surface, as is illustrated in FIG. 2.

Further, the annulus 61 has a portion which is substantially in juxtaposition with a portion of the perimeter of the block of material 17. That is, when the block 17 is cylindrical, which it preferably should be, the perimeter of the block 17 and the annulus 61 are substantially tangential or osculatory eccentric circles. Thus, every portion of the perimeter of the upper surface of the block of material 17 is periodically caused to melt and become a part of the pool of melted material 19 as it passes under, or becomes, the point of tangency of the perimeter and beam 21, thereby preventing the formation of a rim around the perimeter of the block of material 17.

To continuously melt away the perimeter of the block 17 without spilling the pool of melted material 19 over the perimeter, it has been discovered that the volume of the perimeter or outer edge of the block of material 17 melted at any one instant cannot be so large that the inertial and gravitational forces being exerted on it can overcome the cohesive intermolecular forces, or surface tension, exerted on it by the pool of melted material 19. Further, the surface tension will then act to draw the melted perimeter material into the pool of melted material 19.

Accordingly, it follows that the volume of the outer edge melted at any one instant is directly related to the relative lengths of the diameters of the block of material 17 and the annulus 61. That is, as those diameters come closer to being equal, their perimeters come closer to being side by side over their entire lengths and, of course, the entire perimeter comes closer to being molten at all times. Thus, it has been determined that the diameter of the annulus 61 cannot be greater than 97.5 percent of the diameter of the upper surface of the block 17 and preferably should not be greater than 92 percent thereof. Stated differently, the center B of the annulus 61 must at least be offset from the center C of the upper surface of the block 17 by a distance of 2.5 percent of the diameter of the upper surface and preferably should be offset by at least 8 percent thereof.

As is illustrated in FIG. 2, the melted material also forms an eccentric pool 19 within the perimeter of the upper surface of the block of material 17 and overlies nearly the entire upper surface. Partially because of the large diameter of the pool 19 and partially because of the limits on the speed of rotation of the block 17, it has been discovered that the portion of the pool 19 diametrically opposite the center B of the annulus 61 will solidify if at least some portion of the annulus 61 does not extend beyond the center C of the upper surface of the block of material 17. Thus, the diameter of the annulus 61 must at least exceed 50 percent of the diameter of the upper surface of the block 17. Accordingly, the radius of the annulus 61 must exceed 25 percent of the diameter of the upper surface of the block 17, and the center B of the annulus 61 must be offset from the center C of the upper surface of the block 17 by a distance less than 25 percent of the diameter of the upper surface of the block of material 17 and preferably less than 20 percent thereof, since a portion of the annulus 61 must be substantially in juxtaposition with a portion of the perimeter of the upper surface and also extend beyond the center C.

Several factors contribute to the position and flow of the pool of melted material 19 as the block 17 rotates, e.g., centrifugal forces, surface tension, electrical and thermal conductivity, viscosity, and melting temperature of the material in the block 17. It has been found, that when the block -17 is a silicon block, irrespective of its diameter, to avoid spilling the melted material 19 over the edge of the block 17 and to avoid causing too frequent heating of the perimeter, the rate of rotation of the block of material 17 must not exceed 100 revolutions per minute and, preferably, should not exceed 35 revolutions per minute. On the other hand, to keep the pool 19 from freezing while outside of the annulus 61, the block of material 17 must at least make two and preferably should make at least five revolutions per minute.

The use of various heating means to apply heat to one side of the vertical axis of a pedestal charge while rotating that charge to cause the heating effect to be applied to the entire upper surface of the charge has, of course, been shown in the prior art. However, in those prior art methods, a predetermined margin or rim on the upper surface is purposely left unheated to allow the growth of a rim to prevent dripping of any melted material from the upper surface of the pedestal charge. Further, a basic concern in those previously known eccentric heater methods is to heat the upper surface of the charge using as little power as possible. Thus, the configuration of those heating devices generally encloses the minimal area requisite to heating the charge in the desired area and that entire area is raised to at least the melting temperature,

of the material from which a crystal is being grown. Accordingly, when it is desired to add more material or doping material to the melted material, a rod of these materials is merely fed into the melt through the center of the heating device where it is readily melted. Thus, if a seed crystal were similarly fed into the melt, it too would be melted.

On the contrary, in the method of the present invention the center of the thermal field, or the coolest portion of the pool 19, is inside the annulus 61. Thus, the seed crystal is immersed in the pool 19 in the vicinity of the center of the annulus 61, and the seed crystal and growing rod 31 are withdrawn from the pool of melted material 19 at that same location. The seed crystal is drawn at a rate which allows solidification of the melted material uplifted from the pool 19 on the seed crystal. The continued withdrawal of the seed crystal thereby draws or pulls a semiconductor rod from the block 17.

To provide for the continuous growth of a semiconductor rod from the block of material 17, the pedestal 15 can be moved upward while the seed crystal and growing rod 31 are being drawn up from the pool of melted material 19. This causes the upper surface of the block 17 to always be at the same level and allows the annular beam of electrons to be held still while the block 17 is being consumed. Obviously, the beam 21 could be refocused as the upper surface of the block 17 gradually receded if desired. Although other ratios can be used, it has been found that moving the block 17 upward as indicated by the following equation is preferred:

Dc 2 Vb-Vc Db where Vb and Db are respectively the velocity and diameter of the block 17 while V0 and Dc are respectively the velocity and diameter of the growing crystal. These relative movements are achieved, of course, by the conventional means of rotation and reciprocation connected to shafts 16 and 35 as mentioned hereinabove.

Having now described the invention in specific detail and exemplified the manner in which it may be carried into practice, it will be readily apparent to those skilled in the crystal growing art that innumerable variations, applications, modifications and extensions of the basic principles involved may be made without departing from its spirit or scope.

That which is claimed is:

1. In the method of growing a semiconductor rod by positioning in an evacuated container a self-supporting block of material from which said rod is to be grown, melting a portion of said block by directing an electron beam onto the upper surface of said block, forming thereby a pool of said melted material within the perimeter of said upper surface of said block, causing the unmelted portion of said block to serve as a crucible for said melted portion, immersing in said pool a portion of a seed crystal of said material and drawing said seed crystal from said pool, the improvement which comprises evenly melting said upper surface by:

rotating said self-supporting block of material continuously about its vertical axis, causing said pool of melted material formed to be eccentric to said upper surface by directing an annular beam of electrons onto said upper surface so that the center of the annulus of electrons defined by the intersection of said annular beam and said block of material is offset from the center of said upper surface, a first portion of said annular beam of electrons intersects said upper surface substantially tangent to the perimeter of said upper surface, and a second portion of said annular beam of electrons intersects said upper surface diametrically on the opposite side of said center of said upper surface from said center of said annulus,

whereby each point of the perimeter of said upper surface is periodically sequentially exposed to said first portion of said annular beam of electrons and every portion of said upper surface is caused to become a part of said pool of melted material, and

drawing said seed crystal from said pool from within said annulus.

2. The method of growing a semiconductor rod as defined in claim 1, wherein said self-supporting block of material is a substantially cylindrical block.

3. The method of growing a semiconductor rod as defined in claim 2, wherein said block of material is a silicon block continuously rotated at a rate between 2 and 100 revolutions per minute inclusive.

4. The method of growing a semiconductor rod as defined in claim 2, wherein said block of materialis a silicon block continuously rotated at a rate between 5 and 30 revolutions per minute inclusive.

5. The method of growing a semiconductor rod as defined in claim 2, wherein said center of said annulus is olfset from said center of said upper surface by a distance of at least 2.5 percent and less than 25 percent of the diameter of said upper surface.

6. The method of growing a semiconductor rod as defined in claim 2, wherein said center of said annulus is offset from said center of said upper surface by a distance Vb=Vc 2 where Vb and Db are respectively the velocity and diameter of said block of material, and V0 and Dc are respectively the velocity and diameter of the growing rod.

9. The method of growing a semiconductor rod as defined in claim 1, wherein prior to directing said annular beam of electrons onto said upper surface as described therein, the upper surface of said block of material is bombarded with electrons until said upper surface begins to melt.

10. The method of growing a semiconductor rod as defined in claim 1, wherein said annular beam of electrons is emitted by a single electron gun of annular configuration.

References Cited UNITED STATES PATENTS 2,754,259 7/1956 Robinson et al. 23301 2,858,199 10/1958 Larson 23301 3,105,275 10/1963 Hanks 1331 3,228,753 1/1966 Larsen 23301 3,261,722 7/1966 Keller et al 23301 3,278,274 10/ 1966 Liebmann et a1. 23-301 3,342,250 9/1967 Treppschuh et al 13-31 3,360,405 12/1967 Keller 23-301 3,377,419 4/1968 Schiller et a1 1331 3,494,804 2/1970 Hanks et al. 23-301 NORMAN YUDKOFF, Primary Examiner R. T. FOSTER, Assistant Examiner US. 'Cl. X.R. 23--273 SP; 1331 

